Hydroplaning

[Bernoulli]

OK... I have heard from different Airline pilots and other pilots different numbers (all generally close) on the magic number to determine the speed at which an airplane will begin to exhibit dynamic hydroplaning. I have heard "9" times the square root of the main tire pressure "8.7" times the square root of the main tire pressure and even 6.5 times the square root of the main tire pressure....all for takeoff. So... in my handy dandy "Everything Explained for the Professional pilot book by Richie Lengal" on page 372 I find 8.7 times the square root for takeoff and 7.7 times the square root for landing.

Here is my question. I looked at the 3 references that Richie Lengel sites and I can't find any place in those references that actually sites a formula. the references are (AC 91-6A, AC 25-7A, and FAA-H 8083-3 Airplane Flying Handbook) Does anyone know where I can rfind an FAA referance that refers to the types of formulas I have listed above. I'm trying to find out what are the real #'s and have the real source...Also I'm trying to find out if the "formula" is just a rule of thumb that some guy shot out one day and the whole industry has adopted it. Thanks for any constructive comments in advance.

 

[MauleSkinner]

OK... I have heard from different Airline pilots and other pilots different numbers (all generally close) on the magic number to determine the speed at which an airplane will begin to exhibit dynamic hydroplaning. I have heard "9" times the square root of the main tire pressure "8.7" times the square root of the main tire pressure and even 6.5 times the square root of the main tire pressure....all for takeoff. So... in my handy dandy "Everything Explained for the Professional pilot book by Richie Lengal" on page 372 I find 8.7 times the square root for takeoff and 7.7 times the square root for landing.

Here is my question. I looked at the 3 references that Richie Lengel sites and I can't find any place in those references that actually sites a formula. the references are (AC 91-6A, AC 25-7A, and FAA-H 8083-3 Airplane Flying Handbook) Does anyone know where I can rfind an FAA referance that refers to the types of formulas I have listed above. I'm trying to find out what are the real #'s and have the real source...Also I'm trying to find out if the "formula" is just a rule of thumb that some guy shot out one day and the whole industry has adopted it. Thanks for any constructive comments in advance.

If you haven't found it in an "official" FAA document, it may not be there. You could probably find it in a general site on the internet. Hydroplaning is hydroplaning...it'll be the same formula.

Having once been the Upper Midwest's leading expert on hydroplaning (don't ask me why ), I can tell you that 8.7 is the correct number for takeoff (this is the "rotating tire" number), and 7.7 is the correct number for landing (this is the "stationary tire" number). Remember, though, that these are "groundspeeds", not "airspeeds", and that they are in knots, not miles per hour.

Rounding off to 9 and 8 respectively, and rounding tire pressure to an "easy" square number (the square root of 54 isn't all that much different than the square root of 49, which is exactly 7...I like easy math) isn't going to hurt unless you're dealing with airplane speeds that are very close to the actual hydroplaning limit. If you're that close, plan on hydroplaning.

I feel it is also important to emphasize the fact that once hydroplaning starts, it will continue to a MUCH lower speed than is required to trigger it. In other words, if you touch down at 70 knots on tires with a hydroplaning speed at 60 knots, and hydroplaning begins, it may continue well below 60 knots.

I know this isn't the answer you were looking for, but hopefully it is useful.

Fly safe!

David

 

[Hugh Johnson]

[FONT=MSfont-family: Trebuchet,Arial, Helvetica, sans-serif]I think this takes care of it.[/FONT]
[FONT=MSfont-family: Trebuchet,Arial, Helvetica, sans-serif][/FONT]
[FONT=MSfont-family: Trebuchet,Arial, Helvetica, sans-serif][/FONT]
[FONT=MSfont-family: Trebuchet,Arial, Helvetica, sans-serif]"NASA has performed extensive tests and developed a formula that is applicable to all size aircraft. With sufficient water (0.1 inch), hydroplaning will occur at and above the speed predicted by this formula.[/FONT]
NASA critical speed = 9 * (SQRT P)

[FONT=MSfont-family: Trebuchet,Arial, Helvetica, sans-serif]The hydroplaning speed in knots (NASA critical speed) is equal to 9 times the square root of the tire pressure (pounds per square inch)."[/FONT]

 

[Hugh Johnson]

Boy, thats annoying, sorry everyone.

 

[hydroflyer]

I just looked at my Gleim and ASA test prep books, the Gleim uses 8.73 times the square root and ASA uses 9. Also, a Jeppesen book says 8.6, so I guess take your pick. I did notice that if you use 8.73 to get your speed in knots, then using 10 times the square root produces the equivalent speed in mph.

 

[Bernoulli]

I just looked at my Gleim and ASA test prep books, the Gleim uses 8.73 times the square root and ASA uses 9. Also, a Jeppesen book says 8.6, so I guess take your pick. I did notice that if you use 8.73 to get your speed in knots, then using 10 times the square root produces the equivalent speed in mph.

See what I mean? Where is the sourse for these #'s??? They must have come from somewhere. NASA says 9, Gleim says 8.6, ASA says 9, Jeppesen says 8.6. others say 8.7 and 7.7... all of which causes me to think there is no real sourse for the formula or else eveeyone would have the same #'s. Can anyone can direct me to an actual published source recognzed by the FAA or should I just come up with my own numbers...7.89 sound good to me...easy to remember.

 

[cforst513]

does it matter on the amount of water on the runway?

 

[paulsalem]

Or if the tire is bald or has good tred?

 

[landlover]

does it matter on the amount of water on the runway?

i say no, since water (a liquid) will yield to a point stress. it will "move" out of the way of the tire. the only water that will matter is the amount that is inbetween the surface and the contact area of the tire.

 

[MauleSkinner]

Can anyone can direct me to an actual published source recognzed by the FAA or should I just come up with my own numbers...7.89 sound good to me...easy to remember.

No offense, but who cares if the FAA recognizes it? Hydroplaning is hydroplaning, and FAA-recognized hydroplaning isn't any less hazardous than any other kind.

It's a physics issue...the surface tension of the water exerts a pressure against the tire, and the combination of speed and tire pressure determines whether the tire can break the surface tension or not.

As I said earlier, the exact number doesn't make all that much difference...if you have 49 psi tires, the square root is 7. 7x9 (the highest number you found) is 63 knots. 7x8.6 (the lowest "takeoff" number you mentioned) is 60.2 knots. Not enough difference to worry about in the real world.

Underinflated tires are going to make more difference. At 40 psi, the square root would be 6.3, and 6.3x9 is 56 knots, 7 knots less than the "properly inflated" number. I know very few pilots who check the actual tire pressure before flight.

As far as the water depth and tread issues, if there is enough tread to displace the depth of water, hydroplaning is less likely. If you have bald tires, the water can't be displaced as well, and hydroplaning is more likely. If you measure the depth of tread on your airplane prior to flight, and have the tower measure the depth of water, this becomes something you can probably calculate. For the vast majority of us who don't and/or can't get this information, it's simply a "likelihood" issue, but if you can takeoff or land well below the appropriate hydroplaning speed, hydroplaning is highly unlikely.

Fly safe!

David

 

[MauleSkinner]

One additional note...if your tire gets to be more than 6 or 7 degrees off from the direction of travel (either via nosewheel steering or crab), the 8.6 or 9 times the square root is no longer valid...it will act like the 7.7 times the square root of a non-rotating tire.

David

 

[Donsa320]

Or if the tire is bald or has good tred?

Yes it does matter, the water depth must exceed the tread depth, so balding tires will hydroplane before newer ones. None of us would be able to drive in the rain at anything over a crawl if the only factor was the formula mentioned above.

DC